The present invention relates generally to global navigation satellite systems, and more particularly to method and apparatus for reducing the impact of shock and vibration on global navigation satellite system measurements.
Global navigation satellite systems (GNSSs) can determine locations with high accuracy. Currently deployed global navigation satellite systems are the United States Global Positioning System (GPS) and the Russian GLONASS. Other global navigation satellite systems, such as the European GALILEO system, are under development. In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the receiver. The satellite signals comprise carrier signals modulated by pseudo-random binary codes. The receiver measures the time delays of the received signals relative to a local reference clock or oscillator. Code measurements enable the receiver to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges differ from the actual ranges (distances) between the receiver and the satellites due to various error sources and due to variations in the time scales of the satellites and the receiver. If signals are received from a sufficiently large number of satellites, then the measured pseudo-ranges can be processed to determine the code coordinates and coordinate time scales at the receiver. This operational mode is referred to as a stand-alone mode, since the measurements are determined by a single receiver. A stand-alone system typically provides meter-level accuracy.
To improve the accuracy, precision, stability, and reliability of measurements, differential navigation (DN) systems have been developed. In a DN system, the position of a user is determined relative to a base station, also referred to as a base. The base is typically fixed, and the coordinates of the base are precisely known; for example, by surveying. The base contains a navigation receiver that receives satellite signals and that can determine the coordinates of the base by GNSS measurements.
The user, whose position is to be determined, can be stationary or mobile; in a DN system, the user is often referred to as a rover. The rover also contains a navigation receiver that receives satellite signals. Signal measurements processed at the base are transmitted to the rover via a communications link. To accommodate a mobile rover, the communications link is often a wireless link. The rover processes the measurements received from the base, along with measurements taken with its own receiver, to improve the accuracy of determining its position. Accuracy is improved in the differential navigation mode because errors incurred by the receiver at the rover and by the receiver at the base are highly correlated. Since the coordinates of the base are accurately known, measurements from the base can be used to compensate for the errors at the rover. A differential global positioning system (DGPS) computes locations based on pseudo-ranges only.
The location determination accuracy of a differential navigation system can be further improved by supplementing the code pseudo-range measurements with measurements of the phases of the satellite carrier signals. If the carrier phases of the signals transmitted by the same satellite are measured by both the navigation receiver in the base and the navigation receiver in the rover, processing the two sets of carrier phase measurements can yield a location determination accuracy to within several percent of the carrier's wavelength. A differential navigation system that computes locations based on real-time carrier signals, in addition to the code pseudo-ranges, is often referred to as a real-time kinematic (RTK) system. Processing carrier phase measurements to determine coordinates includes the step of ambiguity resolution; that is, determining the integer number of cycles in the carrier signal received by the navigation receiver from an individual satellite.
Global navigation satellite systems have been used for control of construction vehicles, such as bulldozers: an antenna and a receiver are mounted on the vehicle. During operation of the construction vehicle, the antenna and the receiver can be subjected to shock and vibration, which can disturb the GNSS measurements and decrease the accuracy of the coordinate measurements. Method and apparatus for reducing the impact of shock and vibration on GNSS measurements would be advantageous.